The invention relates to an improved method for injecting treatment chemicals into flowing streams and a novel pumping apparatus comprising two or more positive displacement pumps which provides a constant flow of the treatment chemical. Specifically, the injection method comprises the continuous and constant injection of a desired treatment chemical into a flowing stream to insure a uniform concentration of the treatment chemical in the stream. Further, the injection method is accomplished with a pumping apparatus comprising a novel cam in combination with two or more positive displacement pumps. The cam drives the positive displacement pumps so that at any given time the combined rate of liquid discharged by the positive displacement pumps is a constant value.
Conventional methods of injecting treatment chemicals into flowing streams use known positive displacement pumps which provide intermittent and nonconstant flow of the treatment chemicals. The concentration of the treatment chemicals is nonuniform due to the lack of axial dispersion of the treatment chemical in the flowing stream. The nonuniform concentration of the treatment chemical in the stream reduces the desired effect of the treatment chemical.
The hydrocarbon processing industry, chemical industry, oil production industry, water treatment industry, and other similar industries frequently use relatively small amounts of treatment chemicals to control undesirable occurrences in flowing streams in plants. The undesirable occurrences may take many forms such as corrosion, saltation, fouling wax formation, scale formation, and polymerization in pipes or equipment. Corrosion, for example, deteriorates the metal in pipes and process equipment and may cause failure of the pipes or equipment. Likewise, Fouling and wax formation leads to plugging of the pipes or equipment when particular materials are deposited in the pipes and equipment due to undesirable chemical processes.
These problems vary in severity from minor annoyances in the operation of a plant to problems that halt operations of an entire plant. For example, a change from a nonacidic crude oil feedstock in an oil refinery to an acidic crude oil feedstock may cause pipes exposed to the acidic component of the crude oil to experience sudden and severe corrosion. The pipes may develop a hole within hours or days, and cause a processing unit or the whole refinery to shut down. Thus, the effective use of appropriate treatment chemicals to eliminate these problems is of paramount importance to the operation of a hydrocarbon processing plant or other plant.
Various treatment chemicals are available to remedy each of these problems in any particular application. Many chemical companies manufacture and sell treatment chemicals to alleviate specific problems for particular types of flowing streams. For example, Nalco Chemical Number 5192 made by Nalco Chemical Company may be used to prevent corrosion in overhead process streams.
Treatment chemicals are injected intermittently into flowing streams because the pumps used for this purpose provide an intermittent, nonconstant flow of the treatment chemical. Generally, pumps are used for many diverse purposes and many different types of pumps are available for different applications. For example, the chemical and petroleum refining industries use pumps in many applications. Pumps are also used in many everyday settings such as in household appliances and in automobiles. Usually, positive displacement pumps are used to inject treatment chemicals into flowing streams.
Pumps generally fall into two categories: (1) Centrifugal pumps; and (2) positive displacement pumps. Centrifugal pumps operate by applying centrifugal force to a liquid to cause it to flow. In a centrifugal pump liquid is introduced at the center of a rotating member with radial vanes. As the member rotates the liquid is forced to the edge of the member by centrifugal force and discharged.
Centrifugal pumps are the most commonly used type of pump. They are mechanically simple and provide a constant flow of liquid when pumping against a constant pressure. But they are not appropriate in some applications. Specifically, centrifugal pumps are not usually effective when flow rates of 1 gal/min or less are required. Further, centrifugal pumps are not effective for providing a precisely measured amount of liquid because their flow rate is dependent on the pressure they are pumping against. Also, they are not generally useful in applications which require high pressure. Centrifugal pumps have the added disadvantage that they increase the temperature of the fluid being pumped because some of the energy being applied to the fluid does not cause the fluid to move but instead increases the thermal energy of the fluid.
Positive displacement pumps generally operate by using a displaceable member to pull liquid into a chamber and then displace liquid from the chamber. ROBERT H. PERRY AND CECIL H. CHILTON, CHEMICAL ENGINEER'S HANDBOOK, page 6-3 (5th ed. 1973). The chamber of a positive displacement pump is the cavity formed between the displaceable member and the housing of the pump. The volume of the chamber varies as the displaceable member is moved. Many different devices are used to form the chambers and displaceable members of positive displacement pumps.
Positive displacement pumps, in contrast to centrifugal pumps, are ideal for providing a precisely measured flow of liquid. The flow rate delivered by a positive displacement pump depends only on the amount of liquid displaced during a stroke of the displaceable member and the number of strokes of the displaceable member during a given period of time. Further, the pressure that the positive displacement pump is working against has no effect on the flow rate delivered by the pump as it does in centrifugal pumps. Positive displacement pumps are also effective at providing low flow rates because very small displaceable members can be used which provide for a small amount of flow during each stroke of the displaceable member.
Many different types of positive displacement pumps are available. Piston pumps are one type of positive displacement pump. They incorporate a piston as their displaceable member. For example, a Milton-Roy pump incorporates one or more reciprocating pistons in cylinders. See, CHEMICALS ENGINEER'S HANDBOOK, supra at FIG. 6-23. The piston and cylinder form the chamber in which liquid to be pumped is alternately collected and then displaced. The piston pulls liquid into the chamber when the piston is moving in the direction during its stroke which increases the volume of the chamber, and discharges liquid when the piston is moving in the direction during its stroke which decreases the volume of the chamber.
Another type of a positive displacement pump is a diaphragm pump which incorporates a flexible diaphragm as its displaceable member. See, CHEMICAL ENGINEER'S HANDBOOK, supra at FIGS. 6-24 and 6-25. The diaphragm is attached to a housing so that a chamber is formed between the diaphragm and housing. When the diaphragm is flexed away from the chamber liquid is pulled into the chamber, and when the diaphragm is flexed towards the chamber liquid is discharged from the chamber.
In either type of positive displacement pump the cycle of the pump includes two parts: A discharge stroke when liquid is discharged from the chamber and a suction stroke when liquid is pulled into the chamber.
The duration of the discharge stroke and the duration of the suction stroke are the same, and the combined duration for both strokes is the cycle time for the pump. The cycle time for positive displacement pumps ranges from about 0.6 to 1 second. Thus, for a positive displacement pump operating at full capacity, liquid is only being discharged only during 50 percent of the cycle time.
As a result of this type of pump cycle the flow rate of liquid delivered by a positive displacement pump is not constant and stops during the suction stroke. Further, the flow rate delivered by a positive displacement pump during a discharge stroke varies due to the means used to drive the displaceable member of the pump. The flow rate for each displaceable member during the discharge stroke tends to be represented by a sinusoidal wave. See, 1 E. LUDWIG, APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, pages 121-22 (1964). Thus, the flow rate of liquid delivered by positive displacement pumps tends to be intermittent and pulsating. Attempts have been made to overcome this disadvantage by using multiple displaceable members with non-phased cycles so the suction stroke of one member will occur during the discharge of another piston. Id. The effect of adding the sinusoidal discharge rates for multiple out-of-phase displaceable members tends to produce a more constant flow of liquid but does not provide a truly constant flow. Further, these pumps tend to have more mechanical difficulties as the number of displaceable members is increased.
Typically, the amount of liquid discharged by positive displacement pumps may be varied from 10 percent of the pump's discharge capacity to the pump's full discharge capacity. In some positive displacement pumps this is accomplished by adjusting the pump so that it only discharges liquid during a portion of the pump discharge stroke. In other positive displacement pumps the liquid discharged is varied by changing the length of stroke. The result is a decrease in the total amount of liquid discharged by the pump.
The total amount of time that the positive displacement pump does not discharge liquid is the combined amount of time of the suction stroke and the amount of time during the discharge stroke when no liquid is being discharged. Consequently, if the pump is operating at less than full capacity for some positive displacement pumps, treatment chemicals will be injected into the flowing line less than 50 percent of the time.
When no treatment chemical is being discharged by a positive displacement pump the liquid of the flowing stream is continuing to flow past the injection point. This section of liquid is not being treated. With the pump at full capacity the section of liquid with no injected treatment chemical corresponds to the amount of liquid that flows past the injection point during the suction stroke. If the pump is operating at less than full capacity, this section of liquid corresponds to the amount of liquid that flows past the injection point during both the suction stroke and the portion of the discharge stroke when no liquid is discharged. At a minimum, 50 percent of the liquid in the flowing stream will not be injected with treatment chemical. And if the pump is operating at less than full capacity this percentage will be greater than 50 percent.
When treatment chemical is injected intermittently into a flowing stream the chemical will mix rapidly in a radial direction from the point of injection. Consequently, the concentration of the treatment chemical is relatively uniform across the cross-section of the flowing stream within a short distance from the point at which the treatment chemical is injected. This is due to the rapid radial mixing that occurs in the turbulent flow regime of most flowing streams.
Axial mixing, however, does not appear to occur rapidly in a flowing stream. It is generally a function of the nature of the flowing liquid, the nature of the injected liquid, and the flow regime of the flowing liquid. The nature of the flowing liquid and the treatment chemical are important to the extent that the liquids will tend to mix. For example, if the liquids have some chemical attraction to each other they will tend to mix. In the case of a polar treatment chemical being injected into a flowing polar liquid, the polar affinity between the treatment chemical and the flowing liquid will cause axial dispersion more quickly than would occur for a nonpolar treatment chemical injected into a flowing polar liquid.
The flow regime of a flowing fluid is dependent on the velocity of the flowing fluid, the geometry of the flow, and the density and viscosity of the flowing fluid at flow conditions. This relationship is calculated as the Reynold's Number of the flowing fluid. The Reynold's Number is a dimensionless quantity that represents the ratio between the inertial forces in a flowing fluid and the viscous forces in a flowing fluid. It is frequently used to correlate various parameters relating to the behavior of flowing fluids.
The Reynold's Number (Re) for a fluid flowing in a pipe is calculated by the following mathematical formula: EQU Re=DVp/.mu.
where D is the pipe diameter in feet; V is the liquid velocity through the pipe in feet per second; p is the liquid density in pounds per cubic foot; and .mu. is the liquid viscosity in pounds per foot per second. See, CHEMICAL ENGINEER'S HANDBOOK, supra at page 5-4, FIG. 5-26. For a given flow geometry (e.g. flow in a pipe) empirical data related to the Reynold's number indicates whether the flow regime of a flowing liquid is laminar or turbulent.
Laminar flow occurs at low flow velocities, and is characterized by minimal radial mixing on a microscopic scale on the flowing liquid. Further, laminar flow is characterized by different flow velocities for microscopic elements of the flowing liquid depending on the distance between the element of the flowing liquid and the wall of the pipe in which the liquid is flowing. This phenomena occurs because of the frictional forces exerted on the liquid by the pipe wall. Turbulent flow occurs at high flow velocities, and is characterized by extensive radial mixing and random variations in the flow velocities of microscopic elements of the liquid.
For a liquid flowing in a pipe the flow regime is generally laminar at Reynold's Numbers less than 3000, and turbulent at Reynold's Numbers greater than 3000. Typically, flowing streams have Reynold's Numbers in excess of 3000, and the liquids are flowing in a turbulent flow regime.
Reported studies have noted the degree to which axial dispersion will occur in flowing liquids in pipes. T. Sherwood, R. Pigford, and C. Wilke, MASS TRANSFER, McGraw-Hill Publishing Company, 1975, 137-141. These studies generally indicate that axial dispersion of a liquid in another flowing liquid correlates with the Reynolds number of the flowing liquid. MASS TRANSFER, supra at FIG. 4.17. More particularly the effective axial dispersion coefficient, which is a measure of the tendency for a liquid to axially disperse in another flowing liquid, will increase as the Reynold's Number for the flowing liquid increases.
Overall the concentration profile of a liquid injected into a flowing liquid in a turbulent flow regime will follow a Gaussian curve. MASS TRANSFER, supra at 138 and FIG. 4.16. Very little dispersion will occur at a point near the point of injection, and dispersion will gradually increase as the liquid flows farther from the point of injection.
For example, for two batches of oil flowing through a 12-inch pipeline at a velocity of 4 feet per second, the second batch of oil will only be dispersed into the proximate 750 feet of the first batch of oil after traveling 24 miles through the pipeline. MASS TRANSFER, supra at p. 140-41.
Referring to EXAMPLE 1 a test flow loop was constructed to study axial dispersion in a liquid flowing through a tube. Using a diaphragm pump, which provided an intermittent injection of red dye, it was observed that minimal dispersion of the red dye occurred 50 feet from the point of injection of the red dye into a flowing water stream. Further, large sections of the flowing water stream had no observable concentration of the red dye at all.
If this effect is scaled up to the size of typical plant streams it is evident that significant portions of a plant stream will not contain any concentration of a treatment chemical. For example, consider an overhead line in a crude oil processing unit with a 10 inch diameter which carries a flowing liquid with a velocity of 100 feet per second. A positive displacement pump is used to inject a treatment chemical such as a corrosion inhibitor into the overhead zone. The positive displacement pump is operated at 25 percent of its capacity because these pumps are typically sized to provide extra capacity.
If the pump operates at 1 cycle per second and is adjusted to deliver 25% of its capacity the treatment chemical will only be injected for 1/8 of a second. The time period of no injection will be 7/8 of a second. The suction stroke and discharge stroke each last 1/2 second. Treatment chemical is injected during only 25 percent of the discharge stroke or 1/8 second.
During the injection period of 1/8 of a second the flowing stream will move 12.5 feet, and a section 12.5 feet long will contain the treatment chemical. During the period of no injection the flowing stream will move 87.5 feet and a section 87.5 feet long will contain no treatment chemical. Five seconds later the flowing stream will have traveled 500 feet. At which time, based on the flow loop test, the treated section will have slightly expanded from 12.5 feet and the untreated section will have slightly decreased from 87.5 feet.
The combined effect of intermittent injection of a treatment chemical into a flowing stream and the lack of axial dispersion of the treatment chemical in the flowing stream is that significant portions of the flowing stream will have no concentration of the treatment chemical. This problem increases as the velocity of the flowing stream increases relative to the time the pump does not inject treatment chemical because the amount of nontreated flowing stream correspondingly increases. Thus, the effectiveness of the treatment chemical is reduced. In fact, the treatment chemical may not provide any benefit at all under these conditions. Consequently, there is a need for a method that provides a continuous and constant injection of a treatment chemical into a flowing stream and an apparatus for providing a constant flow of the treatment chemical.